The present invention relates to digital transmission and in particular to concepts for reducing the out-of-band radiation of digital transmitters.
In general a digital transmitter comprises an information source, which e.g. might be an MPEG audio encoder for digital radio or an MPEG video encoder for digital television. The output data of the source, which exist in the form of a digital bit stream, are then typically encoded with the aid of a channel encoder so as to introduce the bit stream redundancy which enables transmission errors to be overcome in the receiver. Following this the channel-coded digital bit stream is fed into a so-called xe2x80x9cinterleaverxe2x80x9d, which alters the sequence of the data according to an algorithm which is known to the receiver so that so-called burst errors in the transmission channel do not lead to the loss of a fairly large contiguous part of the message but only to smaller short losses which are spread over a larger time interval. The interleaved bit stream at the output of the interleaver is then imaged in the form of modulation symbols With the aid of a so-called mapper according to the type of modulation used.
If modulation is not employed, and the digital bit stream is effectively transmitted directly without modulation, the mapper and also the modulator which succeeds it can be dispensed with.
If a modulation method is employed, however, e.g. a multicarrier modulation method, the mapper is succeeded by a modulator, which modulates the modulation symbols onto the carrier.
Recently the OFDM method for digital radio applications has become increasingly popular. In this method a plurality of subcarriers is employed onto which the modulation symbols formed by the mapper are modulated. Here the modulation method is an inverse discrete Fourier transform, which, as is known, is used to generate a discrete time signal from the many modulated carriers. The discrete, normally complex time signal exists in the form of complex sampled values or xe2x80x9csamplesxe2x80x9d, which are then fed into an interpolation low-pass in order to remove the periodically repeating spectral contributions. The signal at the output of the interpolation low-pass is typically modulated onto a HF carrier frequency by means of a complex IQ modulator to obtain a HF signal which is fed into a transmitter amplifier which feeds the amplified signal to an antenna which radiates the signal.
In radio applications tube amplifiers such as klystrons or travelling wave tubes are typically used in view of the high power requirements at the output of the amplifier. If smaller powers are acceptable, as in mobile radio e.g., where there is a dense network of transmitters, transistor amplifiers can also be used.
A property which transistor amplifiers and tube amplifiers have in common is that they are linear only over a certain input power range and have an output power curve which falls off at larger input powers and finally levels off at a constant value when the amplifier reaches complete saturation. Put another way, since its characteristic is non-linear, the amplifier causes non-linear distortions of the input signal at higher input powers.
In situations where a certain frequency band has been allocated for a particular transmission application, e.g. through a public licensing authority, regulations stipulate that the transmission signal for the particular licensed transmission application may only carry power within a prescribed band and that there must be no, or very little, power outside the allotted band. The power outside the allotted band is also called out-of-band radiation.
The non-linear characteristic of the amplifier at higher input powers leads, as already mentioned, to non-linear distortions, whose nature is such that the amplifier generates higher harmonics which no longer lie within the allotted band but outside the band and which can be measured as out-to-band radiation.
It is known that these non-linear distortions produce a relatively white spectrum. If the input signal in the amplifier is also band-limited, which is to be expected e.g. for OFDM modulation, the output signal will then possess power outside this band.
To avoid this, i.e. to comply with the licenser""s regulations for the tolerance schemes specified for the frequency bands, i.e. how much out-of-band radiation outside the allotted band is still acceptable, the input voltage into the amplifier should exceed the maximum input voltage for a distortionless amplification as seldom as possible and preferably not at all. In other words this means that the maximum input voltage actually occurring should be as small as possible. If the maximum input voltage is always smaller than the maximum voltage at which the amplifier still operates linearly or only amplifies non-linearly to such a small extent that its out-of-band radiation lies below the permitted value, distortions which lead to an out-of-band radiation which exceeds the permitted value never occur.
A disadvantage of the OFDM method described at the outset is the typically large ratio of peak power to average power, also referred to as PAR (PAR=Peak to Average Power Ratio). Large peaks in the time signal, i.e. in the modulation symbol after the IDFT, can occur if the occupation of the carriers happens to be so unfavourable that e.g. all the 256 OFDM subcarriers overlap constructively at a particular instant. In this case there will be a large signal peak which may easily lie 10 to 20 dB above the average signal power. To comply with the permitted out-of-band radiation despite this, a high power margin, also referred to as xe2x80x9cpower back-offxe2x80x9d, is typically maintained in the transmitter amplifier. Put another way, the amplifier is operated at a working point which is fixed low enough that even a high power peak still falls within the linear range of the amplifier.
This operating mode of an amplifier is an extremely inefficient operating mode in which the amplifier, while requiring considerable supply power, only delivers a relatively small output power. The requirement of low out-of-band radiation in conjunction with high peak values in the time signal, which occur not only for an OFDM modulation but also e.g for a single-carrier method through a pulse shaper, i.e. which can occur generally when filtering, means that expensive amplifiers are needed, which must be operated with a high power margin and which have a low efficiency. However, for smaller battery-operated systems in particular the efficiency is also a point of ever increasing concern, especially with regard to mobile radio and the accumulators with limited storage capacity which are employed there.
WO 98/10567 relates to a method for reducing the peak value factor for digital transmission methods. The basic idea consists in already taking measures on the digital side to ensure that signal peaks which are too high do not occur in the time signal, so that smaller power margins suffice for the transmitter amplifier without resulting in out-of-band radiation which exceeds the permitted limit. The known concept is generally referred to as selected mapping. Selected mapping or SLM really means just that, from a message for transmission, i.e. an information word or more generally speaking a vector of data bits, in some way or other U different possible signals are generated, which can also be termed representatives or candidate transmit sequences. Not all of these signals are transmitted, however. Instead a special signal is selected as the transmission signal. In particular, each transmission signal has a peak value, which is measured. The candidate transmission signal with the lowest peak value is then chosen as the actual transmission signal and is transmitted.
At the receiver the object is now to discover a) what the message is and b) which of the U possible representatives was transmitted per message. There are two possible ways in which the receiver can do this. First, by means of side information, which is communicated to the receiver by the transmitter in some way or other and which relates to which of the U candidate transmit sequences has been chosen. A disadvantage of this method is the explicit transmission of this side information and in particular the fact that this side information must be especially well protected against transmission errors. In the case of a digital radio transmission, where the channel can be disturbed in all sorts of ways and is therefore hard to predict, this is not a trivial matter.
If the side information is wrongly received, the received signal cannot be processed without error. This fact means that this possibility of signalling by means of side information is relatively impracticable.
Another possibility is to effect the concept without the transmission of side information. The receiver must then discover from the received signal alone which message of M messages, i.e. which modulation symbol from a predetermined number of modulation symbols, is present and which of the U candidate transmit sequences was sent per message. Instead of M possible transmission signals or modulation symbols from which the receiver has to pick out the most probable, Mxc2x7U possible transmission signals must be considered in the absence of side information. As a result the error probability can increase considerably in certain circumstances. In addition a suitable choice of the U possible candidate transmit sequences per message is not a trivial problem. To date no suitable solution to this problem has been found, so that transmission without explicit side information is also not yet practicable.
A special variation of SLM which is also described in WO 98/10567 is known as the PTS concept (PTS=Partial Transmit Sequences). The U-candidate transmit sequences are obtained in the following way. Before the transmission signal is finally subjected to linear filtering, e.g. spectral shaping in the case of single-carrier methods or an inverse Fourier transform in the case of OFDM, it is present in the form of a discrete-time signal, i.e. as a vector of elements with complex values. The vector is now partitioned into subsets, i.e. into partial transmit sequences. The elements of each subset can then be multipled by the same complex number of magnitude 1. All these elements are rotated through the same angle in the complex plane. Linear filtering then takes place, through which the high peak values are typically first generated. As a result of the free choice of the complex number which is used for the multiplication, the plurality of candidate transmit sequences can now be generated. Here, too, the two possibilities which have been described above exist, namely whether side information is employed or not. The difference here, however, is that it is not necessary to transmit explicit side information. This is possible because the information to be transmitted is not transmitted absolutely in the complex-value elements of the discrete-time transmission signal but in quotients of successive elements of the same subset. This concept is generally referred to as differential preceding. Since all the elements of a subset have been multiplied by the same complex number, the quotient of two successive elements of the same subset remains the same for each of the U candidate transmit sequences. The receiver must therefore calculate only these quotients to regain the transmitted information.
A disadvantage of the general SLM concept is that it is only practicable in the form it is known at present if side, information is transmitted as well. This is a critical matter, however, in view of the fact that the side information must be specially protected.
Although the PTS concept does not require the transmission of side information, it suffers nevertheless from the disadvantage that it is only practicable in the case of differential precoding. The PTS concept thus considerably curtails the design possibilities for digital transmitters and digital receivers.
Furthermore, measures must be taken in the receiver to perform a power-efficient difference decoding. A receiver must therefore be modified considerably, which may lead to expensive receivers. However, it is precisely in the field of digital radio receivers that the cost of the receivers, which are a mass-produced item, is an important consideration, since even small price advantages can be decisive in consumer markets in determining whether a product survives in the market or not.
The object of the present invention consists in providing a practicable solution to the problem of out-of-band radiation.
In accordance with a first aspect of the present invention, this object is achieved by a device for generating a transmit sequence comprising a unit for forming a plurality of candidate transmit sequences from the information word, each candidate transmit sequence carrying the same information as the information word, where the unit for forming comprises the following features: a unit for adding a label to the information word; a unit for processing the information word plus added label by means of an invertable back-coupled combination algorithm to obtain a combined information word wherein the information units are combined with one another and/or with the added label in such a way that the combined information word is uniquely identified by the label; and a unit for further processing the combined information word to obtain a temporal candidate transmit sequence, where the unit for forming is furthermore so adapted as to generate the plurality of distinct candidate transmit sequences from the same information word with the aid of different labels; and a unit for investigating the plurality of candidate transmit sequences so as to select from the plurality of candidate transmit sequences the candidate transmit sequence which fulfills a predetermined criterion to be the transmit sequence.
In accordance with a second aspect of the present invention, this object is achieved by a device for generating a transmit sequence from an information word with a plurality of information units, comprising: a unit for forming a plurality of candidate transmit sequences from the information word, each candidate transmit sequence carrying the same information as the information word, where the unit for forming comprises the following features: a unit for adding a dummy label to the information word; a unit for processing the information word plus added dummy label by means of an invertable back-coupled combination algorithm to obtain a combined information word wherein the information units are combined with one another and/or with the added dummy label; a unit for linking the combined information word or a word derived from the combined information word with a linking sequence which is determined from a label using the same back-coupled combination algorithm so as to obtain a linked information word which is unambiguously identified by the label; and a unit for further processing the linked information word to obtain a temporal candidate transmit sequence, where the unit for forming is furthermore so adapted as to generate the plurality of distinct candidate transmit sequences from the same information word with the aid of different linking sequences which are determined from different labels; and a unit for investigating the plurality of candidate transmit sequences so as to select from the plurality of candidate transmit sequences the candidate transmit sequence which fulfills a predetermined criterion to be the transmit sequence.
In accordance with a third aspect of the present invention, this object is achieved by a device for determining an information word from a received transmit sequence which has been generated by processing the information word and a label by means of an invertable back-coupled combination algorithm, comprising; a unit for processing the received transmit sequence by means of an inverse algorithm to the invertable back-coupled combination algorithm so as to obtain a sequence which contains the information word and the label; and a unit for extracting the label from the sequence so as to obtain the information word.
In accordance with a fourth aspect of the present invention, this object is achieved by a method for generating a transmit sequence from an information word with a plurality of information units, comprising the steps of: forming a plurality of candidate transmit sequences from the information word, each candidate transmit sequence carrying the same information as the information word, where the step of forming comprises the following substeps: adding a label to the information word; processing the information word plus added label by means of an invertable back-coupled combination algorithm to obtain a combined information word wherein the information units are combined with one another and/or with the added label in such a way that the combined information word is uniquely identified by the label; and further processing the combined information word to obtain a temporal candidate transmit sequence, and where the step of forming is furthermore performed so as to generate the plurality of distinct candidate transmit sequences from the same information word with the aid of different labels; and investigating the plurality of candidate transmit sequences so as to select from the plurality of candidate transmit sequences the candidate transmit sequence which fulfills a predetermined criterion to be the transmit sequence.
In accordance with a fifth aspect of the present invention, this object is achieved by a method for generating a transmit sequence from an information word with a plurality of information units, comprising the steps of: forming a plurality of candidate transmit sequences from the information word, each candidate transmit sequence carrying the same information as the information word, where the step of forming comprises the following substeps: adding a dummy label to the information word; processing the information word plus added dummy label by means of an invertable back-coupled combination algorithm to obtain a combined information word wherein the information units are combined with one another and/or with the added dummy label; linking the combined information word or a word derived from the combined information word with a linking sequence which is determined from a label using the same back-coupled combination algorithm so as to obtain a linked information word which is unambiguously identified by the label; and further processing the linked information word to obtain a temporal candidate transmit sequence, where the step of forming is furthermore performed so as to generate the plurality of distinct candidate transmit sequences from the same information word with the aid of different linking sequences which are determined from different labels; and investigating the plurality of candidate transmit sequences so as to select from the plurality of candidate transmit sequences the candidate transmit sequence which fulfills a predetermined criterion to be the transmit sequence.
In accordance with a sixth aspect of the present invention, this object is achieved by a method for determining an information word from a received transmit sequence which has been generated by processing the information word and a label by means of an invertable back-coupled combination algorithm, comprising the steps of: processing the received transmit sequence by means of an inverse algorithm to the invertable back-coupled combination algorithm so as to obtain a sequence which contains the information word and the label; and extracting the label from the sequence so as to obtain the information word.
The present invention is based on the finding that the problem of the out-of-band radiation can be overcome through a special generation of the candidate transmit sequences.
According to the present invention the generation of the candidate transmit sequences is achieved by means of an invertable back-coupled combination algorithm, which not only processes the information word from which a plurality of candidate transmit sequences are generated, all of which contain the same information as the information word on which they are based, but which also processes a label, the individual candidate transmit sequences being unambiguously identified by the label used in their generation. Back-coupled combination algorithms have the property that they deliver different output values depending on the particular initial conditions in the feedback path of the combination algorithm.
To put the matter another way, for the same input values completely different output vectors are obtained with the same combination algorithm, the different output vectors resulting solely because the combination algorithm with feedback adopts different respective states according to the labels. If the labels appear as a prefix at the start of the information word, the information word is processed starting from a start state of the combination algorithm which is determined by the label. If the label appears elsewhere in the information word, the candidate transmit sequences will not differ until the label is encountered. As soon as the different label enters the combination algorithm, a sort of xe2x80x9cbranchingxe2x80x9d takes place in the combination algorithm, resulting in the various candidate transmit sequences. When account is taken of the fact that an interleaver is normally employed, which performs scrambling anyway, it becomes clear that it is immaterial in the final analysis whether the candidate transmit sequences are identical in a section at the front and differ only in a section at the end.
The label is added to the information word itself, so that both the information word and the label are processed by the back-coupled combination algorithm in such a way that the label is essentially contained implicitly in each candidate transmit sequence.
Put another way, the labels are used to drive the invertable back-coupled combination algorithm into a state, the number of states into which the back-coupled combination algorithm and thus the memory of the combination algorithm can be driven depending on the properties of the feedback path. Since each candidate transmit sequence contains a different label, each candidate transmit sequence is processed by the back-coupled combination algorithm starting from a different state as it were. The label in the candidate transmit sequence is no longer explicitly visible, however. It only reappears if the corresponding candidate transmit sequence in the receiver has been subjected to processing by an algorithm which is the inverse of the algorithm in the transmitter and which thus has a forward coupling property.
After the processing by the inverted back-coupled combination algorithm in the receiver, the label, although it is explicitly present, is no longer needed since it has already done its duty so to speak by driving the forward-coupled combination algorithm, which is inverse to the algorithm in the transmitter, into its appropriate initial state, which forms the basis for the received transmit sequence.
An advantage of the present invention is that it is not necessary to transmit side information explicitly. The label is indeed implicitly transmitted in the candidate transmit sequence, but it does not have to be particularly protected. It should also be noted that normally channel encoding to introduce redundancy in the transmitter and an interleaving operation on the information word.are performed, so that the label is also protected automatically through the same measures as the information word itself without additional effort.
A further advantage of the present invention is that adding the label only results in the addition of a small amount of further redundancy. To give a numerical example: for a concept requiring 16 representatives, a redundancy of only 4 bits is added to an information word of e.g. 512 bits in the case of an OFDM with 256 carriers, which corresponds to a further redundancy of less than 1%, depending on the number of carriers used.
The spectral properties in regard to the transmitter amplifier non-linearities improve as the number of candidate transmit sequences which are generated increases, i.e. the more different states the back-coupled combination algorithm can assume. In general, signal statistics improvements of such an order that the transmitter amplifier power margin can be reduced by values in the order of 2 dB can be achieved with as few as 16 different candidate transmit sequences.
A further advantage of the present invention is that it can be implemented in the receiver with little effort, is robust and thus suitable for practical application.
Another important advantage of the present invention is that the concept according to the present invention is not restricted one or to a small number of modulation types but is generally suitable for diverse forms of channel encoding and for all types of modulation. Generation of the candidate transmit sequences with the aid of an invertable back-coupled combination algorithm thus provides the developer with the maximum design flexibility as regards the type of modulation which is employed, the channel encoding which is employed and the components used in the receiver.